1. Field of the Invention
The invention generally relates to memory technology. In particular, the invention relates to memory devices with a metal-rich metal chalcogenide, such as silver-rich silver selenide.
2. Description of the Related Art
Computers and other digital systems use memory to store programs and data. A common form of memory is random access memory (RAM). Many memory devices, such as dynamic random access memory (DRAM) devices and static random access memory (SRAM) devices are volatile memories. A volatile memory loses its data when power is removed. In addition, certain volatile memories such as DRAM devices require periodic refresh cycles to retain their data even when power is continuously supplied.
In contrast to the potential loss of data encountered in volatile memory devices, nonvolatile memory devices retain data when power is removed. Examples of nonvolatile memory devices include read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), Flash ROM, and the like.
One type of memory device is known as a programmable conductor memory cell or a programmable metallization cell (PMC). See U.S. Pat. Nos. 5,761,115; 5,896,312; 5,914,893; and 6,084,796 to Kozicki, et al., entitled “Programmable metallization cell structure and method of making same,” the disclosures of which are hereby incorporated by reference in their entirety herein. Also see PCT publications WO 00/48196 and WO 02/21542 for additional information. Another term used to describe this cell is a resistance variable material cell. These memory cells can be nonvolatile. A programmable conductor random access memory (PCRAM) includes an array of programmable metallization cells or resistance variable material cells. Additional applications for a programmable metallization cell include use as a programmable resistance and a programmable capacitance.
Information can be stored in a resistance variable material cell by forming or by disrupting conductive pathways, which varies the resistance of the cell. In response to an applied electric field, which can be generated by an electric potential applied between electrodes, a conductive pathway grows from an electrode configured as the cathode, i.e., the electrode with the more negative electric potential, to an electrode configured as the anode, i.e., the electrode with the more positive electric potential. Information can be read or retrieved from the cell by sensing the resistance of the cell.
Conventional processes include techniques that diffuse silver (Ag) through a silver-permeable material, such as a chalcogenide glass. One example of a chalcogenide glass is germanium selenide (GexSe1−x). For example, one conventional process for producing a PMC applies silver (Ag) photodoping to a chalcogenide glass, such as germanium selenide, e.g., Ge3Se7. It is relatively difficult to diffuse silver uniformly across a wafer using conventional techniques. For example, in one conventional process, the doping of the silver varies across the wafer by about 5 to 10%, which can reduce production yield.
Silver-rich silver selenide is relatively difficult to produce using direct deposition techniques. For example, when silver (Ag) is directly deposited on silver selenide (Ag2−δSe), where 0≦δ≦1, in an attempt to create silver-rich silver selenide (Ag2+xSe), the silver (Ag) may agglomerate on the silver selenide and create a rough surface. Similar results may occur when attempting to deposit silver selenide (Ag2Se) directly on silver (Ag).
What is needed is a process to enrich a material, including non-transparent materials, with a metal, such as silver (Ag), to fabricate materials such as silver-rich silver selenide (Ag2+xSe) or silver-rich silver telluride (Ag2+xTe). Such materials can be useful in cell bodies of memory devices.